mutex(fix): prevent race condition (part 2)

In addition to the previous commit, this commit makes threads need to
wait for the thread manager to say it took the mutex from them before
they can ask for it again.

main.cpp

@@ -55,10 +55,13 @@ void *thread_task_increment(const struct thread_data &thread) {
     // mutex
     {
 
-      // tell the thread manager we want the mutex
+      // wait for permission to ask for the mutex
+      while (!*thread.manager_took_mutex)
+        ;
+      // say we want the mutex
       (*thread.wants_mutex) = true;
       // block until we have the mutex
-      while (!*thread.has_mutex && *thread.manager_took_mutex)
+      while (!*thread.has_mutex)
         ;
 
       // enter the mutex